Improving disk cache performance in general is more than just increasing the file system cache size unless your whole system fits the in RAM in which case you should use RAM drive (tmpfs is good because it allows falling back to disk if you need the RAM in some case) for runtime storage (and perhaps an initrd script to copy system from storage to RAM drive at startup).
You didn't tell if your storage device is SSD or HDD. Here's what I've found to work for me (in my case sda is a HDD mounted at /home and sdb is SSD mounted at /).
First optimize the load-stuff-from-storage-to-cache part:
Here's my setup for HDD (make sure AHCI+NCQ is enabled in BIOS if you have toggles):
echo cfq > /sys/block/sda/queue/scheduler
echo 10000 > /sys/block/sda/queue/iosched/fifo_expire_async
echo 250 > /sys/block/sda/queue/iosched/fifo_expire_sync
echo 80 > /sys/block/sda/queue/iosched/slice_async
echo 1 > /sys/block/sda/queue/iosched/low_latency
echo 6 > /sys/block/sda/queue/iosched/quantum
echo 5 > /sys/block/sda/queue/iosched/slice_async_rq
echo 3 > /sys/block/sda/queue/iosched/slice_idle
echo 100 > /sys/block/sda/queue/iosched/slice_sync
hdparm -q -M 254 /dev/sda
Worth noting for the HDD case is high fifo_expire_async (usually write) and long slice_sync to allow a single process to get high throughput (set slice_sync to lower number if you hit situations where multiple processes are waiting for some data from the disk in parallel). The slice_idle is always a compromise for HDDs but setting it somewhere in range 3-20 should be okay depending on disk usage and disk firmware. I prefer to target for low values but setting it too low will destroy your throughput. The quantum setting seems to affect throughput a lot but try to keep this as low as possible to keep latency on sensible level. Setting quantum too low will destroy throughput. Values in range 3-8 seem to work well with HDDs. The worst case latency for a read is (quantum * slice_sync) + (slice_async_rq * slice_async) ms if I've understood the kernel behavior correctly. The async is mostly used by writes and since you're willing to delay writing to disk, set both slice_async_rq and slice_async to very low numbers. However, setting slice_async_rq too low value may stall reads because writes cannot be delayed after reads any more. My config will try to write data to disk at most after 10 seconds after data has been passed to kernel but since you can tolerate loss of data on power loss also set fifo_expire_async to 3600000 to tell that 1 hour is okay for the delay to disk. Just keep the slice_async low, though, because otherwise you can get high read latency.
The hdparm command is required to prevent AAM from killing much of the performance that AHCI+NCQ allows. If your disk makes too much noise, then skip this.
Here's my setup for SSD (Intel 320 series):
echo cfq > /sys/block/sdb/queue/scheduler
echo 1 > /sys/block/sdb/queue/iosched/back_seek_penalty
echo 10000 > /sys/block/sdb/queue/iosched/fifo_expire_async
echo 20 > /sys/block/sdb/queue/iosched/fifo_expire_sync
echo 1 > /sys/block/sdb/queue/iosched/low_latency
echo 6 > /sys/block/sdb/queue/iosched/quantum
echo 2 > /sys/block/sdb/queue/iosched/slice_async
echo 10 > /sys/block/sdb/queue/iosched/slice_async_rq
echo 1 > /sys/block/sdb/queue/iosched/slice_idle
echo 20 > /sys/block/sdb/queue/iosched/slice_sync
Here it's worth noting the low values for different slice settings. The most important setting for an SSD is slice_idle which must be set to 0-1. Setting it to zero moves all ordering decisions to native NCQ while setting it to 1 allows kernel to order requests (but if the NCQ is active, the hardware may override kernel ordering partially). Test both values to see if you can see the difference. For Intel 320 series, it seems that setting slide_idle to 0 gives the best throughput but setting it to 1 gives best (lowest) overall latency.
For more information about these tunables, see http://www.linux-mag.com/id/7572/.
Now that we have configured kernel to load stuff from disk to cache with sensible performance, it's time to adjust the cache behavior:
According to benchmarks I've done, I wouldn't bother setting read ahead via blockdev at all.
Set system to prefer swapping file data over application code (this does not matter if you have enough RAM to keep whole filesystem and all the application code and all virtual memory allocated by applications in RAM). This reduces latency for swapping between different applications over latency for accessing big files from a single application:
echo 15 > /proc/sys/vm/swappiness
Do not ever set this to 0 but if you prefer to keep applications pretty much always in RAM you could set this to 1. If you were memory limited and working with big files (e.g. HD video editing), then it might make sense to set this close to 100.
Next, tell kernel to prefer keeping directory hierarchy in memory over file contents in case some RAM needs to be freed (again, if everything fits in RAM, this setting does nothing):
echo 10 > /proc/sys/vm/vfs_cache_pressure
Setting vfs_cache_pressure to low value makes sense because in most cases, the kernel needs to know the directory structure before it can use file contents from the cache and flushing the directory cache too soon will make the file cache next to worthless. Consider going all the way down to 1 with this setting if you have lots of files. Setting this to big value is sensible only if you have only a few big files that are constantly being re-read.
Finally tell the kernel to use up to 99% of the RAM as cache for writes and instruct kernel to use up to 50% of RAM before slowing down the process that's writing (default for dirty_background_ratio is 10).
echo 99 > /proc/sys/vm/dirty_ratio
echo 50 > /proc/sys/vm/dirty_background_ratio
And tell that 1h write delay is ok to even start writing stuff on the disk:
echo 360000 > /proc/sys/vm/dirty_expire_centisecs
echo 360000 > /proc/sys/vm/dirty_writeback_centisecs
If you put all of those to /etc/rc.local and include following at the end, everything will be in cache as soon as possible after boot:
(nice find / -type f -and -not -path '/sys/*' -and -not -path '/proc/*' -print0 2>/dev/null | nice ionice -c 3 wc -l --files0-from - > /dev/null)&
Or a bit simpler alternative which might work better (cache only /home and /usr):
(nice find /home /usr -type f -print0 | nice ionice -c 3 wc -l --files0-from - > /dev/null)&
